Almost sounds like there might be a need for a website...
Which antigunners would flood with random serial numbers just because.
OP, I used to have a friend on the PD that I would call and he would check the number for me. I could only do this after I had purchased the gun as he couldn't always do it "immediately."
When I purchase a gun, or sell a gun, I check the other person's driver's license, have him sign a bill of sale, I do the same, etc. I've lost some transactions because not everybody will do that. But IMO someone selling a gun they know to be stolen will be reluctant to divulge their name.